This invention relates to a marine engine exhaust assembly, and more particularly to improvements in the external exhaust portions of a stern drive marine engine or the like.
The above-identified U.S. Pat. No. 4,504,238 discloses a typical marine drive including a V-block engine having dual cylinder exhaust elbows which are in turn coupled to an external exhaust system. (Such cylinder exhaust elbows are frequently of the more recent type disclosed in U.S. Pat. No. 4,573,318.) The external system is shown as comprising a pair of upper intermediate elbows which in turn are individually coupled to a lower exhaust pipe, commonly called a "bullhorn". The bullhorn is of generally U-shape and includes a pair of parallel upper pipe legs which merge through bends into a transverse lower pipe portion connected centrally to an exhaust discharge pipe. A baffle or separator plate is shown as being disposed in the lower pipe portion, and takes advantage of centrifugal force to separate cooling water from the exhaust gases traveling therewith. Prior known separator plates have had linear chordal leading edges.
The external exhaust system disclosed in U.S. Pat. No. 4,504,238, as well as other known prior devices of this type, utilizes upper intermediate elbows and bullhorns that are circular in configuration, with each being of generally constant diameter throughout. Furthermore, the exhaust system illustrates one type of "dogleg" therein. That is, a side view of the system would reveal a jogged offset shape, roughly in the form of a Z, formed by each upper elbow in combination with its respective bullhorn leg. Other known doglegs have previously been incorporated entirely in the bullhorn legs themselves.
Although not shown in the aforementioned patents, many marine propulsion exhaust systems include a gimbal attached to the discharge end of the bullhorn, with the separated exhaust and water taking final paths through the gimbal housing in different directions. Due to the relatively massive construction and high weight of such gimbal housings, deep "lightening pockets" have been formed in the housing body to reduce the housing weight. These pockets have been designed to connect with the water discharge passages of the bullhorn, with a final drain holes disposed intermediate the pocket ends for final water discharge out of the housing. The inner ends of the pockets have been generally flat and transverse to the pocket axes.
Engine exhaust back pressure, as measured for example with a manometer in the engine manifold, has always been of concern in the design of marine engines and their exhaust systems. It has been determined that an increase of one psi in back pressure readings results in a decrease in engine horsepower of about 1.5%, which is considered significant to the overall engine performance. Thus, any change in engine and exhaust system construction must be made with the effect on back pressure kept in mind.
Engine exhaust back pressure is greatly influenced by the amount of cooling water induced into the exhaust system, and by how effectively this water is separated from the exhaust gases and ultimately removed from the system. The separators shown in the aforementioned U.S. Pat. No. 4,504,238 are for the purpose of separating the cooling water from the exhaust gases, but it has been noted that at least some water still may flow up and over the tops of the separators into the exhaust gas passages. The presence of this water in the wrong passage is undesirable.
It is an object of the present invention to reduce marine engine exhaust back pressure even further than has been possible heretofore, while maintaining manufacturing efficiencies and costs within acceptable limits.
Broadly in accordance with the various aspects of the invention, a marine engine exhaust assembly is provided with water flow path control means which minimize back pressure. The control means incorporates a plurality of unique "fine tuning" improvements which accumulate to provide what has been found to be a substantial back pressure reduction.
More specifically, a marine engine external exhaust assembly is provided, which is adapted to connect to a pair of engine exhaust elbows which discharge cooling water and exhaust gases thereinto. The assembly includes a pair of intermediate elbows which in turn connect to the upper legs of a bullhorn, with the legs merging through bends into a transverse lower pipe portion. A gas-water shelf-like separator plate is disposed adjacent the bullhorn bends to form individual passages leading to the bullhorn discharge intermediate the ends of the lower pipe section. A gimbal housing is attached to the bullhorn discharge portion.
In accordance with one of the aspects of the invention, the intermediate elbows and bullhorn legs are jointly formed to eliminate the previously known "doglegs". That is, the lower elbow portions and the respective bullhorn upper legs are formed and joined in a manner to provide a continuous common gas-water passage which is axially linear in a fore-to-aft direction.
In accordance with another aspect of the invention, the common gas-water passages are formed to provide a gradual diminishing in cross-sectional area from top to bottom. In the embodiment disclosed herein, the wall contours of the intermediate elbows gradually merge from circular at the upper end to generally elliptical at the lower end. In addition, the wall contours of the upper bullhorn legs are generally elliptical at their upper ends for communicating with the intermediate elbows, and gradually merge into a rectangular shape in the area of the bullhorn bends, which continues on to adjacent the bullhorn discharge portion.
In accordance with yet another aspect of the invention, the leading edges of the gas-water separator plates are formed with a generally V-shaped downstream curve which provides a slicing edge for the water, which is believed to thereby reduce turbulence.
Other aspects of the invention relate to the gimbal housing which has water-carrying lightening pockets which connect from the bullhorn to drain holes in the housing body. In this instance, the lightening pockets have been shortened so that they terminate adjacent their respective drain holes. Furthermore, the inner pocket ends are broadly curved in the direction of the respective drain holes.